Photodiode and photodiode array

ABSTRACT

A p −  type semiconductor substrate  20  has a first principal surface  20   a  and a second principal surface  20   b  opposed to each other and includes a photosensitive region  21 . The photosensitive region  21  is composed of an n +  type impurity region  23 , a p +  type impurity region  25 , and a region to be depleted with application of a bias voltage in the p −  type semiconductor substrate  20 . An irregular asperity  10  is formed in the second principal surface  20   b  of the p −  type semiconductor substrate  20 . An accumulation layer  37  is formed on the second principal surface  20   b  side of the p −  type semiconductor substrate  20  and a region in the accumulation layer  37  opposed to the photosensitive region  21  is optically exposed.

TECHNICAL FIELD

The present invention relates to a photodiode and a photodiode array.

BACKGROUND ART

A photodiode using compound semiconductors is known as a photodiode witha high spectral sensitivity characteristic in the near-infraredwavelength band (e.g., cf. Patent Literature 1). The photodiodedescribed in Patent Literature 1 is provided with a first lightreceiving layer comprised of one of InGaAsN, InGaAsNSb, and InGaAsNP,and a second light receiving layer having an absorption edge at a longerwavelength than that of the first light receiving layer and comprised ofa quantum well structure.

CITATION LIST Patent Literature

-   Patent Literature 1: Japanese Patent Application Laid-open No.    2008-153311

SUMMARY OF INVENTION Technical Problem

However, such photodiodes using the compound semiconductors are stillexpensive and their manufacturing steps are also complicated. For thisreason, there are desires for practical application of a siliconphotodiode being inexpensive and easy to manufacture and havingsufficient spectral sensitivity in the near-infrared wavelength band.The conventional silicon photodiodes generally had the spectralsensitivity characteristic with the limit of about 1100 nm on the longwavelength side, but the spectral sensitivity characteristic in thewavelength band of not less than 1000 nm was not enough.

It is an object of the present invention to provide a silicon photodiodeand a silicon photodiode array as a photodiode and a photodiode arrayhaving a sufficient spectral sensitivity characteristic in thenear-infrared wavelength band.

Solution to Problem

A photodiode according to the present invention is one comprising: asilicon substrate comprised of a semiconductor of a first conductivitytype and having a first principal surface and a second principal surfaceopposed to each other, wherein an avalanche photodiode composed of a pnjunction between a semiconductor region of the first conductivity typehaving a higher impurity concentration than the silicon substrate and asemiconductor region of a second conductivity type is arranged on thefirst principal surface side of the silicon substrate, wherein on thesecond principal surface side of the silicon substrate, an accumulationlayer of the first conductivity type having a higher impurityconcentration than the silicon substrate is formed and an irregularasperity is formed in at least a region opposed to the avalanchephotodiode, and wherein the region opposed to the avalanche photodiodein the second principal surface of the silicon substrate is opticallyexposed.

Since in the photodiode of the present invention the irregular asperityis formed in at least the region opposed to the avalanche photodiode inthe second principal surface, light incident into the photodiode isreflected, scattered, or diffused by the region to travel through a longdistance in the silicon substrate. This causes the light incident intothe photodiode (silicon substrate) to be mostly absorbed in the siliconsubstrate, without passing through the silicon substrate. In theforegoing photodiode, therefore, the travel distance of the lightincident into the photodiode becomes long and the distance of absorptionof light also becomes long, so as to improve the spectral sensitivitycharacteristic in the near-infrared wavelength band.

In the present invention, the accumulation layer of the firstconductivity type having the higher impurity concentration than thesilicon substrate is formed on the second principal surface side of thesilicon substrate. For this reason, unnecessary carriers generatedindependent of light on the second principal surface side recombinethere, so as to reduce dark current. The first conductivity typeaccumulation layer prevents carriers generated by light near the secondprincipal surface of the silicon substrate from being trapped in thesecond principal surface. For this reason, the carriers generated bylight efficiently migrate to the pn junction between the secondconductivity type semiconductor region and the silicon substrate, so asto improve the photodetection sensitivity of the photodiode.

Another photodiode according to the present invention is one comprising:a silicon substrate comprised of a semiconductor of a first conductivitytype, having a first principal surface and a second principal surfaceopposed to each other, and having a semiconductor region of a secondconductivity type formed on the first principal surface side, wherein onthe silicon substrate, an accumulation layer of the first conductivitytype having a higher impurity concentration than the silicon substrateis formed on the second principal surface side and an irregular asperityis formed in at least a region opposed to the semiconductor region ofthe second conductivity type in the second principal surface, andwherein the region opposed to the semiconductor region of the secondconductivity type in the second principal surface of the siliconsubstrate is optically exposed.

In the photodiode according to the present invention, as describedabove, the travel distance of light incident into the photodiode becomeslong and the distance of absorption of light also becomes long, so as toimprove the spectral sensitivity characteristic in the near-infraredwavelength band. The accumulation layer of the first conductivity typeformed on the second principal surface side of the silicon substratereduces the dark current and improves the photodetection sensitivity ofthe photodiode.

In the photodiode according to the present invention, a portion in thesilicon substrate corresponding to the semiconductor region of thesecond conductivity type may be thinned from the second principalsurface side while leaving a surrounding region around the thinnedportion. In this case, the photodiode can be obtained with respectivelight incident surfaces on the first principal surface and secondprincipal surface sides of the silicon substrate.

In the photodiode according to the present invention, preferably, athickness of the accumulation layer of the first conductivity type islarger than a height difference of the irregular asperity. In this case,as described above, it is feasible to ensure the operational effect bythe accumulation layer.

A photodiode array according to the present invention is one comprising:a silicon substrate comprised of a semiconductor of a first conductivitytype and having a first principal surface and a second principal surfaceopposed to each other, wherein a plurality of avalanche photodiodes eachcomposed of a pn junction between a semiconductor region of the firstconductivity type having a higher impurity concentration than thesilicon substrate and a semiconductor region of a second conductivitytype are arranged on the first principal surface side of the siliconsubstrate, wherein on the second principal surface side of the siliconsubstrate, an accumulation layer of the first conductivity type having ahigher impurity concentration than the silicon substrate is formed andan irregular asperity is formed in at least regions opposed to theavalanche photodiodes, and wherein the regions opposed to the avalanchephotodiodes in the second principal surface of the silicon substrate areoptically exposed.

In the photodiode array according to the present invention, as describedabove, the travel distance of light incident into the photodiode arraybecomes long and the distance of absorption of light also becomes long,so as to improve the spectral sensitivity characteristic in thenear-infrared wavelength band. The first conductivity type accumulationlayer formed on the second principal surface side of the siliconsubstrate reduces the dark current and improves the photodetectionsensitivity of the photodiode array.

In the photodiode array according to the present invention, a portion inthe silicon substrate where the plurality of avalanche photodiodes arearranged may be thinned from the second principal surface side whileleaving a surrounding region around the thinned portion. In this case,the photodiode can be obtained with respective light incident surfaceson the first principal surface and second principal surface sides of thesilicon substrate.

In the photodiode array according to the present invention, preferably,a thickness of the accumulation layer of the first conductivity type islarger than a height difference of the irregular asperity. In this case,as described above, it is feasible to ensure the operational effect bythe accumulation layer.

Advantageous Effects of Invention

The present invention provides the silicon photodiode and siliconphotodiode array as the photodiode and the photodiode array with thesufficient spectral sensitivity characteristic in the near-infraredwavelength band.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a drawing for explaining a manufacturing method of aphotodiode according to the first embodiment.

FIG. 2 is a drawing for explaining the manufacturing method of thephotodiode according to the first embodiment.

FIG. 3 is a drawing for explaining the manufacturing method of thephotodiode according to the first embodiment.

FIG. 4 is a drawing for explaining the manufacturing method of thephotodiode according to the first embodiment.

FIG. 5 is a drawing for explaining the manufacturing method of thephotodiode according to the first embodiment.

FIG. 6 is a drawing for explaining the manufacturing method of thephotodiode according to the first embodiment.

FIG. 7 is a drawing for explaining the manufacturing method of thephotodiode according to the first embodiment.

FIG. 8 is a drawing for explaining the manufacturing method of thephotodiode according to the first embodiment.

FIG. 9 is a drawing for explaining the manufacturing method of thephotodiode according to the first embodiment.

FIG. 10 is a drawing for explaining the manufacturing method of thephotodiode according to the first embodiment.

FIG. 11 is a drawing showing a configuration of the photodiode accordingto the first embodiment.

FIG. 12 is a diagram showing changes of spectral sensitivity versuswavelength in Example 1 and Comparative Example 1.

FIG. 13 is a diagram showing changes of temperature coefficient versuswavelength in Example 1 and Comparative Example 1.

FIG. 14 is a drawing for explaining a manufacturing method of aphotodiode according to the second embodiment.

FIG. 15 is a drawing for explaining the manufacturing method of thephotodiode according to the second embodiment.

FIG. 16 is a drawing for explaining the manufacturing method of thephotodiode according to the second embodiment.

FIG. 17 is a drawing for explaining a manufacturing method of aphotodiode according to the third embodiment.

FIG. 18 is a drawing for explaining the manufacturing method of thephotodiode according to the third embodiment.

FIG. 19 is a drawing for explaining the manufacturing method of thephotodiode according to the third embodiment.

FIG. 20 is a drawing for explaining the manufacturing method of thephotodiode according to the third embodiment.

FIG. 21 is a drawing for explaining the manufacturing method of thephotodiode according to the third embodiment.

FIG. 22 is a drawing for explaining a manufacturing method of aphotodiode according to the fourth embodiment.

FIG. 23 is a drawing for explaining the manufacturing method of thephotodiode according to the fourth embodiment.

FIG. 24 is a drawing for explaining the manufacturing method of thephotodiode according to the fourth embodiment.

FIG. 25 is a drawing for explaining a configuration of a photodiodeaccording to the fifth embodiment.

FIG. 26 is a diagram showing changes of spectral sensitivity versuswavelength in Example 2 and Comparative Example 2.

FIG. 27 is a diagram showing changes of spectral sensitivity versuswavelength in Example 2 and Comparative Example 2.

FIG. 28 is a drawing for explaining a configuration of a photodiodeaccording to a modification example of the fifth embodiment.

FIG. 29 is a drawing for explaining a configuration of a photodiodearray according to the sixth embodiment.

FIG. 30 is a drawing for explaining a configuration of the photodiodearray according to the sixth embodiment.

DESCRIPTION OF EMBODIMENTS

The preferred embodiments of the present invention will be describedbelow in detail with reference to the accompanying drawings.

In the description, the same elements or elements with the samefunctionality will be denoted by the same reference signs, withoutredundant description.

First Embodiment

A method for manufacturing a photodiode according to the firstembodiment will be described with reference to FIGS. 1 to 10. FIGS. 1 to10 are drawings for explaining the manufacturing method of thephotodiode according to the first embodiment.

The first step is to prepare an n⁻ type semiconductor substrate 1comprised of silicon (Si) crystal and having a first principal surface 1a and a second principal surface 1 b opposed to each other (cf. FIG. 1).The n⁻ type semiconductor substrate 1 has the thickness of about 300 μmand the resistivity of about 1 kΩ·cm. In the present embodiment, a “highimpurity concentration” refers to, for example, an impurityconcentration of not less than about 1×10¹⁷ cm⁻³ and is denoted by sign“+” attached to conductivity type. A “low impurity concentration” refersto, for example, an impurity concentration of not more than about 1×10¹⁵cm⁻³ and is denoted by sign “−” attached to conductivity type. Examplesof n-type impurities include antimony (Sb), arsenic (As), and so on, andexamples of p-type impurities include boron (B) and others.

Next, a p⁺ type semiconductor region 3 and an n⁺ type semiconductorregion 5 are formed on the first principal surface 1 a side of the n⁻type semiconductor substrate 1 (cf. FIG. 2). The p⁺ type semiconductorregion 3 is formed by diffusing a p-type impurity in a highconcentration from the first principal surface 1 a side in the n⁻ typesemiconductor substrate 1, using a mask opening in a central region. Then⁺ type semiconductor region 5 is formed by diffusing an n-type impurityin a higher concentration than in the n⁻ type semiconductor substrate 1,from the first principal surface 1 a side in the n⁻ type semiconductorsubstrate 1 so as to surround the p⁺ type semiconductor region 3, usinganother mask opening in a peripheral region. The p⁺ type semiconductorregion 3 has the thickness of, for example, about 0.55 μm and the sheetresistance of, for example, 44 Ω/sq. The n⁺ type semiconductor region 5has the thickness of, for example, about 1.5 μm and the sheet resistanceof, for example, 12 Ω/sq.

Next, an insulating layer 7 is formed on the first principal surface 1 aside of the n⁻ type semiconductor substrate 1 (cf. FIG. 3). Theinsulating layer 7 is comprised of SiO₂ and is formed by thermaloxidation of the n⁻ type semiconductor substrate 1. The insulating layer7 has the thickness of, for example, about 0.1 μm. Thereafter, a contacthole H1 is formed in the insulating layer 7 on the p⁺ type semiconductorregion 3 and a contact hole H2 is formed in the insulating layer 7 onthe n⁺ type semiconductor region 5. An antireflective (AR) layercomprised of SiN may be formed instead of the insulating layer 7.

Next, a passivation layer 9 is formed on the second principal surface 1b of the n⁻ type semiconductor substrate 1 and on the insulating layer 7(cf. FIG. 4). The passivation layer 9 is comprised of SiN and is formed,for example, by the plasma CVD process. The passivation layer 9 has thethickness of, for example, 0.1 m. Then the n⁻ type semiconductorsubstrate 1 is polished from the second principal surface 1 b sidebefore the thickness of the n⁻ type semiconductor substrate 1 reaches adesired thickness (cf. FIG. 5). This process removes the passivationlayer 9 from on the second principal surface 1 b of the n⁻ typesemiconductor substrate 1, thereby exposing the n⁻ type semiconductorsubstrate 1. A surface exposed by polishing is also referred to hereinas the second principal surface 1 b. The desired thickness is, forexample, 270 m.

Next, the second principal surface 1 b of the n⁻ type semiconductorsubstrate 1 is subjected to irradiation with a pulsed laser beam PL,thereby forming an irregular asperity 10 (cf. FIG. 6). In this step, asshown in FIG. 7, the n⁻ type semiconductor substrate 1 is placed in achamber C, and the n⁻ type semiconductor substrate 1 is irradiated withthe pulsed laser beam PL from a pulse laser generating device PLDlocated outside the chamber C. The chamber C has a gas inlet port G_(IN)and a gas outlet port G_(OUT) and an inert gas (e.g., nitrogen gas,argon gas, or the like) is introduced through the gas inlet port G_(IN)and discharged through the gas outlet port G_(OUT). This forms an inertgas flow G_(f) in the chamber C. Dust and other materials made duringthe irradiation with the pulsed laser beam PL are discharged as trappedinto the inert gas flow G_(f), to the outside of the chamber C, therebypreventing processing debris, dust, and other materials from attachingto the n⁻ type semiconductor substrate 1.

In the present embodiment, the pulse laser generating device PLD to beused is a picosecond to femtosecond pulse laser generating device and apicosecond to femtosecond pulsed laser beam is applied across the entirearea of the second principal surface 1 b. The second principal surface 1b is roughened by the picosecond to femtosecond pulsed laser beam,whereby the irregular asperity 10 is formed throughout the entire areaof the second principal surface 1 b, as shown in FIG. 8. The irregularasperity 10 has facets intersecting with a direction perpendicular tothe first principal surface 1 a. The height difference of asperity 10is, for example, about 0.5 to 10 μm and the spacing of projections inthe asperity 10 is about 0.5 to 10 μm. The picosecond to femtosecondpulsed laser beam has the pulse duration of, for example, about 50 fs-2ps, the intensity of, for example, about 4 to 16 GW, and the pulseenergy of, for example, about 200 to 800 μJ/pulse. More generally, thepeak intensity is 3×10¹¹ to 2.5×10¹³ (W/cm²) and the fluence is about0.1 to 1.3 (J/cm²). FIG. 8 is an SEM image resulting from observation ofthe irregular asperity 10 formed in the second principal surface 1 b.

Next, an accumulation layer 11 is formed on the second principal surface1 b side of the n⁻ type semiconductor substrate 1 (cf. FIG. 9). In thisstep, the accumulation layer 11 is formed by ion implantation ordiffusion of an n-type impurity from the second principal surface 1 bside in the n⁻ type semiconductor substrate 1 so that an impurityconcentration thereof becomes higher than that of the n⁻ typesemiconductor substrate 1. The accumulation layer 11 has the thicknessof, for example, about 1 μm.

Next, the n⁻ type semiconductor substrate 1 is subjected to a thermaltreatment (annealing). In this step, the n⁻ type semiconductor substrate1 is heated, for example, in the temperature range of about 800 to 1000°C. under an ambiance of N₂ gas for about 0.5 to 1 hour.

Next, the passivation layer 9 formed on the insulating layer 7 isremoved and thereafter electrodes 13, 15 are formed (cf. FIG. 10). Theelectrode 13 is formed in the contact hole H1 and the electrode 15 inthe contact hole H2. The electrodes 13, 15 each are comprised ofaluminum (Al) or the like and have the thickness of, for example, about1 μm. This completes the photodiode PD1.

The photodiode PD1 is provided with the n⁻ type semiconductor substrate1, as shown in FIG. 10. The p⁺ type semiconductor region 3 and the n⁺type semiconductor region 5 are formed on the first principal surface 1a side of the n⁻ type semiconductor substrate 1 and a pn junction isformed between the n⁻ type semiconductor substrate 1 and the p⁺ typesemiconductor region 3. The electrode 13 is in electrical contact withand connection to the p⁺ type semiconductor region 3 through the contacthole H1. The electrode 15 is in electrical contact with and connectionto the n⁺ type semiconductor region 5 through the contact hole H2.

The irregular asperity 10 is formed in the second principal surface 1 bof the n⁻ type semiconductor substrate 1. The accumulation layer 11 isformed on the second principal surface 1 b side of the n⁻ typesemiconductor substrate 1 and the second principal surface 1 b isoptically exposed. That the second principal surface 1 b is opticallyexposed encompasses not only the case where the second principal surface1 b is in contact with ambient gas such as air, but also the case wherean optically transparent film is formed on the second principal surface1 b.

In the photodiode PD1, the irregular asperity 10 is formed in the secondprincipal surface 1 b. For this reason, light L incident into thephotodiode PD1 is reflected, scattered, or diffused by the asperity 10,as shown in FIG. 11, to travel through a long distance in the n⁻ typesemiconductor substrate 1.

Normally, Si has the refractive index n=3.5 and air the refractive indexn=11.0. In a photodiode, when light is incident from a direction normalto a light incident surface, light remaining unabsorbed in thephotodiode (silicon substrate) is separated into a light componentreflected on the back surface to the light incident surface and a lightcomponent passing through the photodiode. The light passing through thephotodiode does not contribute to the sensitivity of the photodiode. Thelight component reflected on the back surface to the light incidentsurface, if absorbed in the photodiode, becomes a photocurrent. A lightcomponent still remaining unabsorbed is reflected or transmitted by thelight incident surface as the light component having reached the backsurface to the light incident surface was.

In the photodiode PD1, where light L is incident from the directionnormal to the light incident surface (first principal surface 1 a), whenthe light reaches the irregular asperity 10 formed in the secondprincipal surface 1 b, light components arriving thereat at angles ofnot less than 16.6° to a direction of emergence from the asperity 10 aretotally reflected by the asperity 10. Since the asperity 10 is formedirregularly, it has various angles to the emergence direction and thetotally reflected light components diffuse into various directions. Forthis reason, the totally reflected light components include lightcomponents absorbed inside the n⁻ type semiconductor substrate 1 andlight components arriving at the first principal surface 1 a and sidefaces.

The light components arriving at the first principal surface 1 a andside faces travel in various directions because of the diffusion on theasperity 10. For this reason, the light components arriving at the firstprincipal surface 1 a and the side faces are extremely highly likely tobe totally reflected on the first principal surface 1 a and the sidefaces. The light components totally reflected on the first principalsurface 1 a and the side faces are repeatedly totally reflected ondifferent faces to further increase their travel distance. In thismanner, the light L incident into the photodiode PD1 is absorbed in then⁻ type semiconductor substrate 1 during travel through the longdistance inside the n⁻ type semiconductor substrate 1 to be detected asa photocurrent.

As described above, the light L incident into the photodiode PD1 mostlytravels, without being transmitted by the photodiode PD1, through thelong travel distance to be absorbed in the n⁻ type semiconductorsubstrate 1. Therefore, the photodiode PD1 is improved in the spectralsensitivity characteristic in the near-infrared wavelength band.

If a regular asperity is formed in the second principal surface 1 b, thelight components arriving at the first principal surface 1 a and theside faces are diffused by the asperity but travel in uniformdirections. Therefore, the light components arriving at the firstprincipal surface 1 a and the side faces are less likely to be totallyreflected on the first principal surface 1 a and the side faces. Thisresults in increase in light passing through the first principal surface1 a and the side faces, and through the second principal surface 1 b,and thus the travel distance of the light incident into the photodiodemust be short. As a result, it becomes difficult to improve the spectralsensitivity characteristic in the near-infrared wavelength band.

An experiment was conducted in order to check the effect of improvementin the spectral sensitivity characteristic in the near-infraredwavelength band by the first embodiment.

We fabricated a photodiode with the above-described configuration(referred to as Example 1) and a photodiode without the irregularasperity in the second principal surface of the n⁻ type semiconductorsubstrate (referred to as Comparative Example 1), and investigated theirspectral sensitivity characteristics. Example 1 and Comparative Example1 have the same configuration, except for the formation of the irregularasperity by irradiation with the pulsed laser beam. The size of the n⁻type semiconductor substrate 1 was set to 6.5 mm×6.5 mm. The size of thep⁺ type semiconductor region 3, or a photosensitive region was set to5.8 mm×5.8 mm. A bias voltage VR applied to the photodiodes was set to 0V.

The results are shown in FIG. 12. In FIG. 12, the spectral sensitivitycharacteristic of Example 1 is represented by T1 and the spectralsensitivity characteristic of Comparative Example 1 by characteristicT2. In FIG. 12, the vertical axis represents the spectral sensitivity(mA/W) and the horizontal axis the wavelength of light (nm). Acharacteristic indicated by a chain line represents a spectralsensitivity characteristic where the quantum efficiency (QE) is 100%,and a characteristic indicated by a dashed line, a spectral sensitivitycharacteristic where the quantum efficiency is 50%.

As seen from FIG. 12, for example at 1064 nm, the spectral sensitivityin Comparative Example 1 is 0.2 A/W (QE=25%) whereas the spectralsensitivity in Example 1 is 0.6 A/W (QE=72%); thus the spectralsensitivity in the near-infrared wavelength band is drasticallyimproved.

We also checked temperature characteristics of spectral sensitivity inExample 1 and Comparative Example 1. We investigated the spectralsensitivity characteristics with increase in ambient temperature from25° C. to 60° C. and calculated a rate (temperature coefficient) ofspectral sensitivity at 60° C. to spectral sensitivity at 25° C.

The results are shown in FIG. 13. In FIG. 13, the characteristic oftemperature coefficient of Example 1 is represented by T3 and that ofComparative Example 1 by characteristic T4. In FIG. 13, the verticalaxis represents the temperature coefficient (%/° C.) and the horizontalaxis the wavelength of light (nm).

As seen from FIG. 13, for example at 1064 nm, the temperaturecoefficient in Comparative Example 1 is 0.7%/° C., whereas thetemperature coefficient in Example 1 is 0.2%/° C., demonstrating lowertemperature dependence. In general, an increase in temperature leads toan increase in spectral sensitivity because of increase in absorptioncoefficient and decrease in bandgap energy. In Example 1, since thespectral sensitivity is sufficiently high even at room temperature, thechange of spectral sensitivity due to temperature rise is smaller thanin Comparative Example 1.

In the photodiode PD1, the accumulation layer 11 is formed on the secondprincipal surface 1 b side of the n⁻ type semiconductor substrate 1.This induces recombination of unnecessary carriers generated independentof light on the second principal surface 1 b side, which can reduce darkcurrent. The accumulation layer 11 prevents carriers generated by lightnear the second principal surface 1 b, from being trapped in the secondprincipal surface 1 b. For this reason, the carriers generated by lightefficiently migrate to the pn junction, which can further improve thephotodetection sensitivity of the photodiode PD1.

In the first embodiment, after the formation of the accumulation layer11, the n⁻ type semiconductor substrate 1 is subjected to the thermaltreatment. This treatment restores the crystallinity of the n⁻ typesemiconductor substrate 1, which can prevent such a problem as increaseof dark current.

In the first embodiment, after the thermal treatment of the n⁻ typesemiconductor substrate 1, the electrodes 13, 15 are formed. Thisprevents the electrodes 13, 15 from melting during the thermaltreatment, even in the case where the electrodes 13, 15 are made of ametal with a relatively low melting point. As a result, the electrodes13, 15 can be appropriately formed without being affected by the thermaltreatment.

In the first embodiment, the irregular asperity 10 is formed by theirradiation with the picosecond to femtosecond pulsed laser beam. Thispermits the irregular asperity 10 to be appropriately and readilyformed.

Second Embodiment

A method for manufacturing a photodiode according to the secondembodiment will be described with reference to FIGS. 14 to 16.

FIGS. 14 to 16 are drawings for explaining the manufacturing method ofthe photodiode according to the second embodiment.

The manufacturing method of the second embodiment, up to the polishingof the n⁻ type semiconductor substrate 1 from the second principalsurface 1 b side, is the same as the manufacturing method of the firstembodiment, and the description of the previous steps before it isomitted herein. After the n⁻ type semiconductor substrate 1 is polishedfrom the second principal surface 1 b side to obtain the n⁻ typesemiconductor substrate 1 in the desired thickness, the accumulationlayer 11 is formed on the second principal surface 1 b side of the n⁻type semiconductor substrate 1 (cf. FIG. 14). The formation of theaccumulation layer 11 is carried out in the same manner as in the firstembodiment. The accumulation layer 11 has the thickness of, for example,about 1 μm.

Next, the second principal surface 1 b of the n⁻ type semiconductorsubstrate 1 is irradiated with the pulsed laser beam PL to form theirregular asperity 10 (cf. FIG. 15). The formation of the irregularasperity 10 is carried out in the same manner as in the firstembodiment.

Next, as in the first embodiment, the n⁻ type semiconductor substrate 1is subjected to a thermal treatment. Thereafter, the passivation layer 9formed on the insulating layer 7 is removed and then the electrodes 13,15 are formed (cf. FIG. 16). This completes the photodiode PD2.

In the second embodiment, as in the first embodiment, the traveldistance of light incident into the photodiode PD2 also becomes long andthe distance of absorption of light also becomes long. This allows thephotodiode PD2 also to be improved in the spectral sensitivitycharacteristic in the near-infrared wavelength band.

In the second embodiment, the thickness of the accumulation layer 11 islarger than the height difference of the irregular asperity 10. For thisreason, even if the irregular asperity 10 is formed by the irradiationwith the pulsed laser beam after the formation of the accumulation layer11, the accumulation layer 11 remains with certainty. Therefore, it isfeasible to ensure the operational effect by the accumulation layer 11.

Third Embodiment

A method for manufacturing a photodiode according to the thirdembodiment will be described with reference to FIGS. 17 to 21. FIGS. 17to 21 are drawings for explaining the manufacturing method of thephotodiode according to the third embodiment.

The manufacturing method of the third embodiment, up to the formation ofthe passivation layer 9, is the same as the manufacturing method of thefirst embodiment, and the description of the previous steps before it isomitted herein. After the formation of the passivation layer 9, aportion corresponding to the p⁺ type semiconductor region 3 in the n⁻type semiconductor substrate 1 is thinned from the second principalsurface 1 b side while leaving a surrounding region around the thinnedportion (cf. FIG. 17). The thinning of the n⁻ type semiconductorsubstrate 1 is carried out, for example, by anisotropic etching based onalkali etching using a potassium hydroxide solution, TMAH(tetramethylammonium hydroxide solution), or the like. The thinnedportion of the n⁻ type semiconductor substrate 1 has the thickness of,for example, about 100 μm, and the surrounding region around it has thethickness of, for example, about 300 μm.

Next, the n⁻ type semiconductor substrate 1 is polished from the secondprincipal surface 1 b side before the thickness of the surroundingregion of the n⁻ type semiconductor substrate 1 reaches a desiredthickness (cf. FIG. 18). The desired thickness herein is, for example,270 μm.

Next, the second principal surface 1 b of the n⁻ type semiconductorsubstrate 1 is irradiated with the pulsed laser beam PL to form theirregular asperity 10 (cf. FIG. 19). The formation of the irregularasperity 10 is carried out in the same manner as in the firstembodiment.

Next, the accumulation layer 11 is formed on the second principalsurface 1 b side of the thinned portion of the n⁻ type semiconductorsubstrate 1 (cf. FIG. 20). The formation of the accumulation layer 11 iscarried out in the same manner as in the first embodiment. Theaccumulation layer 11 has the thickness of, for example, about 3 μm.

Next, as in the first embodiment, the n⁻ type semiconductor substrate 1is subjected to a thermal treatment and thereafter, the passivationlayer 9 formed on the insulating layer 7 is removed, followed byformation of the electrodes 13, 15 (cf. FIG. 21). This completes thephotodiode PD3.

In the third embodiment, as in the first and second embodiments, thetravel distance of light incident into the photodiode PD3 also becomeslong and the distance of absorption of light also becomes long. Thisallows the photodiode PD3 also to be improved in the spectralsensitivity characteristic in the near-infrared wavelength band.

In the third embodiment, prior to the formation of the irregularasperity 10, the portion corresponding to the p⁺ type semiconductorregion 3 in the n⁻ type semiconductor substrate 1 is thinned from thesecond principal surface 1 b side while leaving the surrounding regionaround the thinned portion. This permits the photodiode PD3 to be formedwith respective light incident surfaces on the first principal surface 1a and the second principal surface 1 b sides of the n⁻ typesemiconductor substrate 1.

Fourth Embodiment

A method for manufacturing a photodiode according to the fourthembodiment will be described with reference to FIGS. 22 to 24. FIGS. 22to 24 are drawings for explaining the manufacturing method of thephotodiode according to the fourth embodiment.

The manufacturing method of the fourth embodiment, up to the thinning ofthe n⁻ type semiconductor substrate 1, is the same as the manufacturingmethod of the third embodiment, and the description of the previoussteps before it is omitted herein. After the n⁻ type semiconductorsubstrate 1 is polished from the second principal surface 1 b side toobtain the n⁻ type semiconductor substrate 1 in the desired thickness,the accumulation layer 11 is formed on the second principal surface 1 bside of the thinned portion of the n⁻ type semiconductor substrate 1(cf. FIG. 22). The formation of the accumulation layer 11 is carried outin the same manner as in the first embodiment. The accumulation layer 11has the thickness of, for example, about 3 μm.

Next, the second principal surface 1 b of the n⁻ type semiconductorsubstrate 1 is irradiated with the pulsed laser beam PL to form theirregular asperity 10 (cf. FIG. 23). The formation of the irregularasperity 10 is carried out in the same manner as in the firstembodiment.

Next, the n⁻ type semiconductor substrate 1 is subjected to a thermaltreatment as in the first embodiment. Then the passivation layer 9formed on the insulating layer 7 is removed and thereafter, theelectrodes 13, 15 are formed (cf. FIG. 24). This completes thephotodiode PD4.

In the fourth embodiment, as in the first to third embodiments, thetravel distance of light incident into the photodiode PD4 also becomeslong and the distance of absorption of light also becomes long.

This allows the photodiode PD4 also to be improved in the spectralsensitivity characteristic in the near-infrared wavelength band.

In the fourth embodiment, prior to the formation of the accumulationlayer 11, the portion corresponding to the p⁺ type semiconductor region3 in the n⁻ type semiconductor substrate 1 is thinned from the secondprincipal surface 1 b side while leaving the surrounding region aroundthe thinned portion. This permits the photodiode PD4 to be formed withrespective light incident surfaces on the first principal surface 1 aand the second principal surface 1 b sides of the n⁻ type semiconductorsubstrate 1.

Fifth Embodiment

A photodiode PD5 according to the fifth embodiment will be describedwith reference to FIG. 25. FIG. 25 is a drawing for explaining aconfiguration of the photodiode of the fifth embodiment.

The photodiode PD5 is an avalanche photodiode for detecting low-energylight the wavelength region of which is in the visible to near infraredregion. The photodiode PD5 is provided with a p⁻ type semiconductorsubstrate 20. The p⁻ type semiconductor substrate 20 is comprised ofsilicon (Si) crystal and has a first principal surface 20 a and a secondprincipal surface 20 b opposed to each other. The p⁻ type semiconductorsubstrate 20 includes a photosensitive region 21.

The photosensitive region 21 is disposed in a central region of thefirst principal surface 20 a on a plan view. The photosensitive region21 has the thickness inward from the first principal surface 20 a. Thephotosensitive region 21 is composed of an n⁺ type impurity region 23, ap⁺ type impurity region 25, and a region that is depleted withapplication of a bias voltage in the p⁻ type semiconductor substrate 20.The n⁻ type impurity region 23 has the thickness inside the p⁻ typesemiconductor substrate 20 from the first principal surface 20 a. The n⁻type impurity region 23 has an n⁻ type guard ring 23 a. The n⁻ typeguard ring 23 a is provided at the peripheral edge of the n⁻ typeimpurity region 23. The p⁺ type impurity region 25 has the thicknessfurther inside the p⁻ type semiconductor substrate 20 from the n⁺ typeimpurity region 23. The p⁻ type semiconductor substrate 20 has a p⁺ typediffusion blocking region 27. The p⁺ type diffusion blocking region 27is disposed at the peripheral edge of the first principal surface 20 aon the plan view and has the thickness inward from the first principalsurface 20 a. The p⁺ type diffusion blocking region 27 is provided so asto surround the photosensitive region 21.

The p⁻ type semiconductor substrate 20 is a silicon substrate doped witha p-type impurity, e.g., such as boron (B). The p⁺ type impurity region25 is a region doped with a p-type impurity in a higher concentrationthan the p⁻ type semiconductor substrate 20. The p⁺ type diffusionblocking region 27 is a region doped with a p-type impurity in a higherconcentration than the p⁺ type impurity region 25. The n⁺ type impurityregion 23 is a region doped with an n-type impurity, e.g., such asantimony (Sb). The n⁺ type impurity region 23 (including the n⁺ typeguard ring 23 a) and the p⁺ type impurity region 25 constitute a pnjunction in the p⁻ type semiconductor substrate 20.

The photodiode PD5 has a passivation film 29 deposited on the firstprincipal surface 20 a. The photodiode PD5 has an electrode 31 and anelectrode 33 disposed on the passivation film 29. In the passivationfilm 29, a contact hole H11 is provided on the n⁺ type impurity region23 and a contact hole H12 is provided on the p⁺ type diffusion blockingregion 27. The electrode 31 is in electrical contact with and connectionto the n⁺ type impurity region 23 through the contact hole H11. Theelectrode 33 is in electrical contact with and connection to the p⁺ typediffusion blocking region 27 through the contact hole H12. A material ofthe passivation film 29 is, for example, silicon oxide or the like.

The photodiode PD5 has a recess 35 formed on the second principalsurface 20 b side. The recess 35 is formed by thinning the p typesemiconductor substrate 20 from the second principal surface 20 b sideand a thick frame portion exists around the recess 35. The side face ofthe recess 35 is inclined at an obtuse angle relative to the bottom faceof the recess 35. The recess 35 is formed so as to overlap thephotosensitive region 21 on the plan view. The thickness between thebottom face of the recess 35 and the first principal surface 20 a isrelatively small, e.g., about 100-200 μm, and is preferably about 150μm. Since the thickness between the first principal surface 20 a and thebottom face of the recess 35 is relatively small as described above, theresponse speed becomes higher and the bias voltage applied to thephotodiode PD5 is reduced.

The irregular asperity 10 is formed throughout the entire secondprincipal surface 20 b of the p⁻ type semiconductor substrate 20. Anaccumulation layer 37 is formed on the second principal surface 20 bside of the p⁻ type semiconductor substrate 20. In the accumulationlayer 37, a region corresponding to the bottom face of the recess 35,i.e., the region opposed to the photosensitive region 21 forming theavalanche photodiode is optically exposed. That the second principalsurface 20 b is optically exposed embraces, not only the case where thesecond principal surface 20 b is in contact with ambient gas such asair, but also the case where an optically transparent film is formed onthe second principal surface 20 b. The irregular asperity 10 may beformed only in the bottom face of the recess 35, i.e., only in theregion opposed to the photosensitive region 21 functioning as theavalanche photodiode.

The photodiode PD5 has an electrode 39. The electrode 39 is provided onthe accumulation layer 37 and is in electrical contact with andconnection to the accumulation layer 37. The region where the electrode39 is formed in the accumulation layer 37 is not optically exposed.

In the photodiode PD5 having the above configuration, when a reversebias voltage (breakdown voltage) is applied to the electrode 31 and theelectrode 39, carriers according to the quantity of light incident intothe photosensitive region 21 are generated in the photosensitive region21. The carriers generated near the p⁺ type diffusion blocking region 27flow into the p⁺ type diffusion blocking region 27. For this reason, thep⁺ type diffusion blocking region 27 reduces a tail in an output signalfrom the electrode 31.

The following will describe a method for manufacturing the photodiodePD5 of the fifth embodiment.

First, the p⁻ type semiconductor substrate 20 is prepared. The thicknessof the p⁻ type semiconductor substrate 20 is about 300 m.

Next, the p⁺ type impurity region 25 and p⁺ type diffusion blockingregion 27 are formed on the first principal surface 20 a side of the p⁻type semiconductor substrate 20. The p⁺ type impurity region 25 isformed by ion implantation of a p-type impurity in a high concentrationfrom the first principal surface 20 a side in the p⁻ type semiconductorsubstrate 20, using a mask opening in a central region. The p⁺ typediffusion blocking region 27 is formed by diffusing a p-type impurity ina high concentration from the first principal surface 20 a side in thep⁻ type semiconductor substrate 20, using another mask opening in aperipheral region.

Next, the n⁺ type guard ring 23 a and the n⁺ type impurity region 23 areformed on the first principal surface 20 a side of the p⁻ typesemiconductor substrate 20. The n⁺ type guard ring 23 a is formed bydiffusing an n-type impurity in a high concentration from the firstprincipal surface 20 a side in the p⁻ type semiconductor substrate 20,using a mask opening in a ring shape. The n⁺ type impurity region 23 isformed by ion implantation of an n-type impurity in a high concentrationfrom the first principal surface 20 a side in the p⁻ type semiconductorsubstrate 20, using another mask opening in a central region.

Next, the surface of the second principal surface 20 b of the p typesemiconductor substrate 20 is planarized by polishing. Thereafter, aportion corresponding to the p⁺ type impurity region 25 in the p⁻ typesemiconductor substrate 20 is thinned from the second principal surface1 b side while leaving a surrounding region around the thinned portion.The thinning of the p⁻ type semiconductor substrate 20 is carried out byanisotropic etching, e.g., alkali etching using a KOH aqueous solution,TMAH, or the like. The thickness of the thinned portion of the p⁻ typesemiconductor substrate 20 is, for example, about 150 vim and thethickness of the surrounding region is, for example, about 200 μm.

Next, the accumulation layer 37 is formed on the second principalsurface 20 b side of the p⁻ type semiconductor substrate 20. Here, theaccumulation layer 37 is formed by ion implantation of a p-type impurityin a higher impurity concentration than in the p⁻ type semiconductorsubstrate 20, from the second principal surface 20 b side in the p⁻ typesemiconductor substrate 20. The thickness of the accumulation layer 37is, for example, about 1.5 μm.

Next, the p⁻ type semiconductor substrate 20 is subjected to a thermaltreatment (anneal). Here, the p⁻ type semiconductor substrate 20 isheated in the temperature range of about 900 to 1100° C., morepreferably about 1000° C., in an ambiance such as N₂ gas for about 0.5to 1.0 hour, more preferably for about 0.5 hour. The thermal treatmentrestores the crystallinity of the p⁻ type semiconductor substrate 20 andthus prevents the problem such as increase of dark current.

Next, the second principal surface 20 b of the p⁻ type semiconductorsubstrate 20 is irradiated with a pulsed laser beam PL to form theirregular asperity 10. The irregular asperity 10 is formed byirradiating the second principal surface 20 b of the p typesemiconductor substrate 20 with the pulsed laser beam, as in theaforementioned embodiments. A pulse laser generating device for theirradiation with the pulsed laser beam can be a picosecond tofemtosecond pulse laser generating device. The irregular asperity 10 hasfacets intersecting with the direction perpendicular to the firstprincipal surface 20 a. The height difference of the asperity 10 is, forexample, about 0.5-10 μm and the spacing of projections in the asperity10 is about 0.5-10 μm. The picosecond to femtosecond pulsed laser beamhas the pulse duration of, for example, about 50 fs-2 ps, the intensityof, for example, about 4-16 GW, and the pulse energy of, for example,about 200-800 J/pulse. More generally, the peak intensity is about3×10¹¹ to 2.5×10¹³ (W/cm²) and the fluence about 0.1 to 1.3 (J/cm²).

Next, the p⁻ type semiconductor substrate 20 is subjected to a thermaltreatment (anneal). Here, the p⁻ type semiconductor substrate 20 isheated in the temperature range of about 900 to 1100° C., morepreferably about 1000° C., in an ambiance such as N₂ gas, for about0.5-1.0 hour, more preferably for about 0.5 hour. The thermal treatmentachieves recovery from damage of disordered crystal andrecrystallization thereof.

Next, the passivation film 29 is formed on the first principal surface20 a side of the p⁻ type semiconductor substrate 20. Then the contactholes H11, H12 are formed in the passivation film 29 and the electrodes31, 33 are formed. The electrode 31 is formed in the contact hole H11and the electrode 33 in the contact hole H12.

Furthermore, the electrode 39 is formed on the accumulation layer 37 inthe surrounding region around the thinned portion of the p⁻ typesemiconductor substrate 20. The electrodes 31, 33 each are comprised ofaluminum (Al) or the like and the electrode 39 is comprised of gold (Au)or the like. This completes the photodiode PD5.

In the photodiode PD5, the irregular asperity 10 is formed in the secondprincipal surface 20 b. For this reason, the light incident into thephotodiode PD5 is reflected, scattered, or diffused by the asperity 10to travel through a long distance in the p⁻ type semiconductor substrate20.

In the photodiode PD5, where the light is incident from the directionnormal to a light incident surface (first principal surface 20 a), whenthe light arrives at the irregular asperity 10 formed in the secondprincipal surface 20 b, light components arriving thereat at angles ofnot less than 16.6° relative to the direction of emergence from theasperity 10 are totally reflected by the asperity 10. Since the asperity10 is formed irregularly, it has various angles relative to theemergence direction and diffuses the totally reflected light componentsinto various directions. For this reason, the totally reflected lightcomponents include light components absorbed inside the p⁻ typesemiconductor substrate 20 and light components reaching the firstprincipal surface 20 a and side faces.

The light components reaching the first principal surface 20 a and theside faces travel in various directions because of the diffusion at theasperity 10. For this reason, the light components reaching the firstprincipal surface 20 a and the side faces are extremely highly likely tobe totally reflected by the first principal surface 20 a and the sidefaces. The light components totally reflected by the first principalsurface 20 a and the side faces are repeatedly totally reflected ondifferent faces, whereby the travel distance thereof further increases.Therefore, the light incident into the photodiode PD5 is absorbed by thep⁻ type semiconductor substrate 20 during the travel through the longdistance inside the p⁻ type semiconductor substrate 20 to be detected asa photocurrent.

The light L incident into the photodiode PD5 mostly travels, withoutbeing transmitted by the photodiode PD5, through the long traveldistance to be absorbed in the p⁻ type semiconductor substrate 20.Therefore, the photodiode PD5 is improved in the spectral sensitivitycharacteristic in the near-infrared wavelength band.

An experiment was conducted in order to check the improvement effect inthe spectral sensitivity characteristic in the near-infrared wavelengthband by the fifth embodiment.

We produced the photodiode having the above-described configuration(which is referred to as Example 2) and a photodiode without theirregular asperity in the second principal surface of the p typesemiconductor substrate (which is referred to as Comparative Example 2)and checked the spectral sensitivity characteristics of the respectivephotodiodes. Example 2 and Comparative Example 2 have the sameconfiguration except for the formation of the irregular asperity byirradiation with the pulsed laser light. The size of the p⁻ typesemiconductor substrate 20 was set to 4.24 mm×4.24 mm. The size of thep⁺ type impurity region 25, i.e., the size of the photosensitive regionwas set to 3 mmφ. The bias voltage VR applied to the photodiodes was setto about 300 V.

The results are provided in FIG. 26. In FIG. 26, the spectralsensitivity characteristic of Example 2 is indicated by T5 ₁ and thespectral sensitivity characteristic of Comparative Example 2 bycharacteristic T5 ₂. In FIG. 26, the vertical axis represents thespectral sensitivity (mA/W) and the horizontal axis the wavelength oflight (nm). As seen from FIG. 26, for example at 1064 nm, the spectralsensitivity is 4.1 A/W in Comparative Example 2, whereas the spectralsensitivity is 7.6 A/W in Example 2, confirming that the spectralsensitivity is drastically improved in the near-infrared wavelengthband.

In the photodiode PD5, the accumulation layer 37 is formed on the secondprincipal surface 20 b side of the p⁻ type semiconductor substrate 20.This induces recombination of unnecessary carriers generated independentof light on the second principal surface 20 b side, so as to reduce thedark current. The accumulation layer 37 prevents carriers generated bylight near the second principal surface 20 b, from being trapped in thesecond principal surface 20 b. For this reason, the carriers generatedby light efficiently migrate to the pn junction, so as to furtherimprove the photodetection sensitivity of the photodiode PD5.

In the fifth embodiment, the p⁻ type semiconductor substrate 20 issubjected to the thermal treatment, after formation of the accumulationlayer 37. This restores the crystallinity of the p⁻ type semiconductorsubstrate 20, so as to prevent the problem such as increase of darkcurrent.

The accumulation layer 37 may be formed after formation of the irregularasperity 10. In the case where the irregular asperity 10 is formed byirradiation with the pulsed laser light, after the formation of theaccumulation layer 37, the thickness of the accumulation layer 37 ispreferably set to be larger than the height difference of the irregularasperity 10. In this case, the accumulation layer 37 is certainly lefteven after the irregular asperity 10 is formed by irradiation with thepulsed laser light. The operational effect by the accumulation layer 37can be ensured accordingly.

In the fifth embodiment, the electrodes 31, 33, 39 are formed after thethermal treatment of the p⁻ type semiconductor substrate 20. Thisprevents the electrodes 31, 33, 39 from melting during the thermaltreatment, even if the electrodes 31, 33, 39 are made of materials witha relatively low melting point. Therefore, the electrodes 31, 33, 39 canbe appropriately formed without being affected by the thermal treatment.

In the fifth embodiment, the irregular asperity 10 is formed byirradiation with the picosecond to femtosecond pulsed laser beam. Thisallows the irregular asperity 10 to be appropriately and readily formed.

In the fifth embodiment, the p⁻ type semiconductor substrate 20 isthinned from the second principal surface 20 b side. This allows us toobtain the photodiode with the respective light incident surfaces on thefirst principal surface 20 a side and the second principal surface 20 bside of the p⁻ type semiconductor substrate 20. Namely, the photodiodePD5 can be used, not only as a front-illuminated type photodiode, butalso as a back-thinned type photodiode.

We conducted an experiment for checking the improvement effect in thespectral sensitivity characteristic in the near-infrared wavelength bandin the case where the photodiode PD5 of the fifth embodiment was used asa back-thinned type photodiode.

We checked the respective spectral sensitivity characteristics withincidence of light from the back surface, using the aforementionedphotodiodes of Example 2 and Comparative Example 2. The results areprovided in FIG. 27. In FIG. 27, the spectral sensitivity characteristicof Example 2 is indicated by T5 ₃ and the spectral sensitivitycharacteristic of Comparative Example 2 by characteristic T5 ₄. In FIG.27, the vertical axis represents the spectral sensitivity (mA/W) and thehorizontal axis the wavelength of light (nm). As seen from FIG. 27, forexample at 1064 nm, the spectral sensitivity is 1.9 A/W in ComparativeExample 2, whereas the spectral sensitivity is 5.7 A/W in Example 2,confirming that the spectral sensitivity is drastically improved in thenear-infrared wavelength band.

As described above, the photodiode PD5 of the fifth embodiment has thesufficient spectral sensitivity at 1064 nm, regardless of whether it isused as the front-illuminated type or the back-thinned type. Therefore,the photodiode PD5 can be used as a device for detecting YAG laserlight.

Incidentally, concerning the avalanche photodiode, it is possible torealize the avalanche photodiode with a practically sufficient spectralsensitivity characteristic in the near-infrared wavelength band, bysetting the semiconductor substrate of silicon thick (e.g., aboutseveral hundred μm to 2 mm). However, the avalanche photodiodenecessitates the bias voltage for depletion and the bias voltage foravalanche multiplication and, therefore, in the case where the thicknessof the semiconductor substrate is set large, it becomes necessary toapply the extremely high bias voltage. Furthermore, the thicksemiconductor substrate also increases the dark current.

In the photodiode PD5 of the fifth embodiment, however, since theirregular asperity 10 is formed in the second principal surface 20 b asdescribed above, the travel distance of the light incident into thephotodiode PD5 is lengthened. For this reason, it is feasible to realizethe photodiode with the practically sufficient spectral sensitivitycharacteristic in the near-infrared wavelength band, without need forincreasing the thickness of the semiconductor substrate (p⁻ typesemiconductor substrate 20), particularly, the portion corresponding tothe photosensitive region 21. Therefore, the foregoing photodiode PD5can achieve the good spectral sensitivity characteristic withapplication of a lower bias voltage than the photodiode with thespectral sensitivity characteristic in the near-infrared wavelength bandbased on the increase in the thickness of the semiconductor substrate.In addition, the increase of dark current is suppressed, so as toimprove the detection accuracy of the photodiode PD5. Furthermore, sincethe thickness of the p⁻ type semiconductor substrate 20 is small, theresponse speed of the photodiode PD5 improves.

In the photodiode PD5 of the fifth embodiment, the entire region on thesecond principal surface 20 b side may be thinned as shown in FIG. 28.

Sixth Embodiment

A photodiode array PDA according to the sixth embodiment will bedescribed with reference to FIG. 29. FIG. 29 is a drawing for explaininga cross-sectional configuration of the photodiode array according to amodification example of the sixth embodiment.

The photodiode array PDA is provided with a p⁻ type semiconductorsubstrate 20, and a plurality of photosensitive regions 21 functioningas avalanche photodiodes are arranged on the p⁻ type semiconductorsubstrate 20.

The irregular asperity 10 is formed throughout the entire secondprincipal surface 20 b of the p⁻ type semiconductor substrate 20.Namely, the photodiode array PDA has the irregular asperity 10 formednot only in regions opposed to the photosensitive regions 21 functioningas avalanche photodiodes, but also in regions opposed to the regionsbetween the photosensitive regions 21.

In the sixth embodiment, as in the fifth embodiment, the travel distanceof light incident into the photodiode array PDA also becomes long andthe distance of absorption of light also becomes long. This allows thephotodiode array PDA also to be improved in the spectral sensitivitycharacteristic in the near-infrared wavelength band.

The photodiode array PDA of the sixth embodiment, like the fifthembodiment, can achieve the good spectral sensitivity characteristicwith application of a lower bias voltage than a photodiode array with apractically sufficient spectral sensitivity characteristic in thenear-infrared wavelength band based on the increase in the thickness ofthe semiconductor substrate. In addition, the increase of dark currentis suppressed, so as to improve the detection accuracy of the photodiodearray PDA. Furthermore, since the thickness of the p⁻ type semiconductorsubstrate 20 is small, the response speed of the photodiode array PDAimproves.

In the photodiode array PDA, the irregular asperity 10 is also formed inthe regions opposed to the regions between the photosensitive regions 21in the second principal surface 20 b of the p⁻ type semiconductorsubstrate 20. For this reason, light L incident between thephotosensitive regions 21 is reflected, scattered, or diffused by theirregular asperity 10 formed in the regions opposed to the regionsbetween the photosensitive regions 21 in the second principal surface 20b, as shown in FIG. 30, to be absorbed by any one of the photosensitiveregions 21. In the photodiode array PDA, therefore, the detectionsensitivity is not lowered between the photosensitive regions 21, so asto improve the detection sensitivity.

The photodiode array PDA can also be used as a device for detecting YAGlaser light as the photodiode PD5 of the fifth embodiment.

In the photodiode array PDA the entire region on the second principalsurface 20 b side may be thinned as in the case of the photodiode PD5 ofthe fifth embodiment. The photodiode array PDA can be used as aphotodiode array of either of the front-illuminated type and theback-thinned type.

The above described the preferred embodiments of the present invention,but it should be noted that the present invention is not always limitedto the above-described embodiments and that the present invention can bemodified in many ways without departing from the spirit and scope of theinvention.

In the first to fourth embodiments the irregular asperity 10 is formedby irradiating the entire area of the second principal surface 1 b withthe pulsed laser beam, but it is not limited only to this example. Forexample, the irregular asperity 10 may be formed, for example, byirradiating only a region opposed to the p⁺ type semiconductor region 3in the second principal surface 1 b of the n⁻ type semiconductorsubstrate 1 with the pulsed laser beam. In the fifth and sixthembodiments the irregular asperity 10 may also be formed by irradiatingonly the region opposed to each photosensitive region 21 with the pulsedlaser beam.

In the first to fourth embodiments the electrode 15 is in electricalcontact with and connection to the n⁺ type semiconductor region 5 formedon the first principal surface 1 a side of the n⁻ type semiconductorsubstrate 1, but it is not limited only to this example. For example,the electrode 15 may be in electrical contact with and connection to theaccumulation layer 11 formed on the second principal surface 1 b side ofthe n⁻ type semiconductor substrate 1. In this case, the electrode 15 ispreferably formed outside the region opposed to the p⁺ typesemiconductor region 3 in the second principal surface 1 b of the n⁻type semiconductor substrate 1. The reason for it is as follows: if theelectrode 15 is formed in the region opposed to the p⁺ typesemiconductor region 3 in the second principal surface 1 b of the n⁻type semiconductor substrate 1, the irregular asperity 10 formed in thesecond principal surface 1 b is blocked by the electrode 15, causing anevent of reduction in the spectral sensitivity in the near-infraredwavelength band. The above-described matter also applies to the fifthand sixth embodiments.

The conductivity types of p type and n type in the photodiodes PD1-PD5and the photodiode array PDA in the embodiments may be interchanged soas to be reverse to those described above.

INDUSTRIAL APPLICABILITY

The present invention is applicable to semiconductor photodetectionelements and photodetection apparatus.

LIST OF REFERENCE SIGNS

1 n⁻ type semiconductor substrate; 1 a first principal surface; 1 bsecond principal surface; 3 p⁺ type semiconductor region; 5 n⁺ typesemiconductor region; 10 irregular asperity; 11 accumulation layer; 13,15 electrodes; 20 p⁻ type semiconductor substrate; 20 a first principalsurface; 20 b second principal surface; 21 photosensitive region; 23 n⁺type impurity region; 25 p⁺ type impurity region; 37 accumulation layer;PL pulsed laser beam; PD1-PD5 photodiodes; PDA photodiode array.

1-7. (canceled) 8: A photodiode array comprising: a silicon substratecomprised of a semiconductor of a first conductivity type and having afirst principal surface and a second principal surface opposed to eachother, wherein a plurality of avalanche photodiodes each composed of apn junction between a semiconductor region of the first conductivitytype having a higher impurity concentration than the silicon substrateand a semiconductor region of a second conductivity type are arranged onthe first principal surface side of the silicon substrate, wherein onthe second principal surface side of the silicon substrate, anaccumulation layer of the first conductivity type having a higherimpurity concentration than the silicon substrate is formed and anirregular asperity is formed in at least regions opposed to theavalanche photodiodes, wherein the regions opposed to the avalanchephotodiodes in the second principal surface of the silicon substrate areoptically exposed, and wherein the second principal surface where theirregular asperity is formed in at least the regions opposed to theavalanche photodiodes constitutes a light incident surface, lightincident from the second principal surface travels in the siliconsubstrate, the photodiode being a back-illuminated type. 9: Thephotodiode array according to claim 8, wherein in the silicon substrate,a portion where the plurality of avalanche photodiodes are arranged isthinned from the second principal surface side while leaving asurrounding region around said thinned portion. 10: The photodiode arrayaccording to claim 8, wherein a thickness of the accumulation layer ofthe first conductivity type is larger than a height difference of theirregular asperity. 11: The photodiode array according to claim 8,wherein the light incident from the second principal surface andtraveling in the silicon substrate is reflected, scattered, or diffusedby the irregular asperity. 12: The photodiode array according to claim8, wherein the irregular asperity is formed by applying pulsed laserbeam.